The problems associated with prior art systems for separating higher and lower boiling constituents or a mixture thereof is best illustrated by the separation of natural gas. Natural gas, as it is received from a subsurface formation, generally is not suitable for use directly, without some processing. The initial processing operations carried out in a natural gas plant are to first remove acid gases, such as CO.sub.2 and H.sub.2 S and then pass the gas through a dehydration system to remove water. Thus, it would be advantageous if one of these preliminary treatment steps, such as the removal of CO.sub.2 could be eliminated while still producing acceptable products. The resulting product can then be used as a fuel. However, such natural gases generally contain significant amounts of higher molecular weight hydrocarbons, such as ethane and to a lesser extent, propane, butanes and higher molecular weight hydrocarbons. The ethane and higher molecular weight hydrocarbons contribute relatively little heating value to the natural gas and have a significantly greater value as chemical feedstocks than as a fuel. Therefore, it is also highly advantageous to maximize the recovery of ethane and higher molecular weight hydrocarbons.
The natural gas feed to a natural gas plant will generally be at about atmospheric temperature and at an elevated pressure substantially above atmospheric pressure, either as received from the producing formation or having been compressed. Therefore, it has long been known to separate ethane and higher molecular weight hydrocarbons from methane by a combination of plural cooling stages and an expansion stage and separate the cooled and expanded fluid in a demethanizer to produce a vapor stream substantially enriched in methane content and a liquid stream substantially enriched in ethane and higher molecular weight hydrocarbons. However, such systems are not particularly efficient. Accordingly, it has been proposed in the past to improve the efficiency by utilizing two or more expansion stages in series. Even with such multiple expansion stages in a series, separation is still rather inefficient and difficult. First, the demethanizer either must be rather large and/or a given size demethanizer is limited in its throughput capacity. Secondly, the amounts of ethane retained in the separated methane stream is generally higher than desirable. The heat required by the demethanizer is rather high, thus, reducing the energy efficiency of the plant. The energy generated by the expanders is usually inadequate to handle all of the pressure requirements and the refrigeration needs of the overall plant. Finally, if CO.sub.2 is not removed from the gas by prior treatment, amounts of CO.sub.2 in the ethane and higher molecular weight hydrocarbon fraction are higher than desirable.